Significance of the measurement of uric acid fractional clearance in diuretic induced hyponatraemia.

The biochemical features of severe hyponatraemia due to thiazide administration in 7 non-oedematous patients were compared with those in hyponatraemia due to frusemide. Hypouricaemia has been shown to occur in hyponatraemia due to the syndrome of inappropriate antidiuretic hormone activity and this was measured along with fractional uric acid clearances in all the patients. Five of the patients had been on thiazides (or hydrochlorothiazide with amiloride) for only a few days to a few weeks. Fractional uric acid clearance was elevated and serum uric acid levels were low in five of them and returned to the normal range on restoration of serum sodium to normal. By contrast, the patients on frusemide did not show any abnormality in fractional uric acid clearance at any stage. These results are consistent with excess ADH activity as having caused hyponatraemia induced by thiazides in 5 of the 7 cases, whereas frusemide caused a sodium depletion syndrome. Treatment in the former cases is by water restriction, and in frusemide-induced salt depletion by saline supplementation.